Method of production of a dispenser cathode



Sept. 27, 1966 R. H. BRlsToW ETAL 3,275,435

METHOD OF PRODUCTION OF A DISPENSER CATHODE Filed Nov.

INVENTORS JOHN H. AFFLECKJII. ROBERT H.BRISTOW,

BY y

THEIR ATTORNEY.

United States Patent 3,275,435 METHOD OF PRODUCTION OF A DISPENSERCATHODE Robert H. Bristow, Burnt Hills, and John H. Afileck III,

Schenectady, N.Y., assignors to General Electric Company, a corporationof New York Filed Nov. 1, 1963, Ser. No. 320,725 7 Claims. (Cl. 75-201)This invention relates to refractory metal dispenser cathodesincorporating a softer additive metal, and more particularly to a hightemperature method of compacting dispenser cathode material at reducedpressures.

A dispenser cathode may be defined as a combination of a refractorymetal and a compound containing an activator cathode material. Theactivator cathode material is dispensed to the surface of the cathode ina continuous process and thus maintaining an emissive surface by theactivator. One suitable combination is, for example, tungsten as thematrix and a barium compound as the activator cathode material. It isgenerally known that electron tubes are limited in life and performanceby the specific cathode employed. An examination of general factorslimiting the frequency and power obtainable from electron tubes alsoreveals that cathode inadequacies are the most important of theselimitations.

One cathode limitation is referred to as dissipation which is the resultof loss of activator cathode material. This loss is referred to as theevaporation rate of the activator material. Basically, the factors thatdetermine the evaporation rate may be divided into two categories: (1)the rate of production of the activating agent from the activatorcathode material such as Ba and BaO from a barium compound; and (2) thetransport mechanism by which the active cathode material is dispensedthrough the matrix and over the emitting surface. The transport of theactivator material, for example barium, may be controlled through thepore size or porosity of the matrix material. The porosity and pore sizeof a dispenser matrix material is governed to a considerable extent bothby the particle size and the compacting pressure. Optimum porosity forrefractory metal dispenser cathodes require very high compactingpressures and very high sintering temperatures. These conditions aredetrimental to the overall production of dispenser cathodes and furtherdetrimental to the more precise predetermination of porosity.

Accordingly, it is an object of this invention to provide an improveddispenser cathode.

It is another object of this invention to provide an improved method ofcompacting and sintering a dispenser cathode.

It is a frmther object of this invention to provide for the addition ofpredetermined additive materials to reduce high pressures and hightemperatures associated with dispenser cathode production.

Briefly described, thistinvention in one form comprises the addition ofsmall aniounts of a Group VIII metal, preferably nickel, in elemental orcombined form dispersed throughout a refractory metal which is in powderor small particle form. Upon compressing or compacting the mixture, theadditive nickel is operative to reduce the required compacting pressureand facilitate the predetermination of optimum porosity. Sinteringtemperature of the matrix is also lowered.

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This invention will be better understood when taken in connection withthe following description and the drawing in which:

The figure is a cross-sectional illustration of one apparatus in whichthe present invention may be carried out.

Various methods are known in the art by which dispenser cathodes may beproduced. For example, the known cold pressed method includes mixing arefractory metal powder and a barium compound and pressing to thedesired shape at pressures of about tons per square inch. Thereafter thecompact is heated at a temperature of about 1800 C. in vacuum or in aredducing atmosphere for sintering to provide an integral strongcohesive structure.

A further method of producing a dispenser cathode includes pressing arefractory metal powder, for example tungsten, at about 70 tons persquare inch and then sintering the matrix in a reducing atmosphere suchas dry hydrogen at about 2500 C. The matrix is then impregnated withmolten copper at about 1400 C. The c0pper-filled matrix is machined tothe desired shape and the copper removed by firing at about 2000 C. invacuum. Thereafter, the pores or interstices of the tungsten matrix arefilled with a molten barium compound by hot firing at about 1700" C. ina vacuum or reducing atmosphere.

A further method of producing dispenser cathodes is disclosed andclaimed in co-pending application Ser. No. 320,724 assigned to the sameassignee as the present invention and filed concurrently herewith. Inthe mentioned co-pending application the method disclosed includes acombination step of simultaneously pressing and heating a predeterminedmix for a dispenser cathode. This combination step is carried out, inone form of the invention, in a carbon crucible to provide a protectiveatmosphere.

In any of the above-mentioned methods the required pressures may begreatly reduced by the addition of small amounts of one or more of themetals taken from class consisting of those metals of Group VIII of theperiodic table of elements. Preferentially these metals include nickel,cobalt, palladium, platinum, ruthenium and rhodium. Best results havebeen obtained in the practice of this invention where the additive metalcontains nickel or cobalt and where the additive metal is of commercialpurity.

The additive nickel should be widely dispersed as much as possiblethroughout the refractory metal matrix for optimum results. Best resultsare obtained when the amount of additive nickel is maintained at aminimum value and where the amount does not add substantial elementalnickel characteristics to the dispenser cathode. For example, in thepractice of this invention in one form, the amount of nickel added isless than 5% and ordinarily about 1% by weight of the refractory metalto be employed. The added amount should be sufiicient to provide atleast the weight percent (0.3) which will go into solid solution.

The additive nickel may be added by various means known in the art,whether by elemental nickel or through the use of materials or compoundscontaining nickel which react or otherwise decompose to provideelemental nickel under the conditions of the pressing and sintering. Byway of example, such compounds may include nickelous nitrate, nickeloxide, nickel acetate, etcetera.

In one preferred form of this invention, in order to add the requiredamount of nickel, and in order to obtain excellent dispersion of thenickel, a solution of nickelous nitrate in alcohol was added to microntungsten powder in such a quantity that 1% by weight of nickel withrespect to the matrix material was distributed throughout the matrix.

The effect of the nickel is to reduce the compressing and sinteringtemperature of the matrix material. The nickel element does notcompletely line the pores or openings in the refractory material nor isit dispersed in such a manner as to add its particular characteristicsto dispenser operation. Of the various metals chosen for the attainmentof the given purposes, it has been discovered that nickel combines thesalient features of being easily dispersed within a refractory metalmatrix and cooperates therewith to reduce the required pressures andtemperatures for compacting, and does not chemically or electronicallyenter into the ordinary dispenser operation to detract therefrom. To thecontrary, the effectiveness of the cathode is increased through theaddition of nickel. Comparable examples indicate a change in density ofthe cathode from the former 60% without nickel to 90% with nickel.Nickel coats the tungsten particles so that upon sintering an intimatebond is obtained. This bond is acquired at pressures of about tons persquare inch compared to former employed pressures on the order of 70tons per square inch.

A representative example of the practice of this invention included adispenser cathode which consisted essentially of tungsten powder ofabout 5 micron particle size together with one weight percent nickel toprovide an 88% by weight refractory matrix. To this powdered matrixthere was added 10% by weight barium orthosilicate (Ba 'SiO 1% by weightcalcium carbonate (CaCO and 1% by weight zirconium hydride (ZrH therebytotaling 100% by weight dispenser cathode. After thoroughly mixing thesematerials a compact was produced therefrom by pressing the mixture atabout 10 tons per square inch and thereafter firing the compact in dryhydrogen at about 1300 C. for about 30 minutes. Dispenser cathodes weremachined from the resulting compact and emission tested with goodresults.

In conjunction with the above representative example and the practicesas set forth in the above-mentioned copending application, best resultsare obtained by the practice of this invention utilizing the othermaterials and proportions in the example such as Ba SiO CaCO ZrI-I Theabove example is representative of a method of producing an optimumcathode carried out in accordance with the teachings of this invention.This process has been repetitiously employed to produce many cathodes.It is to be understood, however, that various other mixtures both as topercentage mixtures and as to materials may be employed within the scopeof this invention. For example, nickel may be added from about 0.3weight percent to about 1 weight percent. Nickel in much greater amountsis unnecessary and should be avoided where high temperature operation ofthe cathode is contemplated because of its higher vapor pressure. It ispreferred to maintain the additive material, nickel, in sufficientamounts to provide about 0.3 weight percent nickel in the tungstenmatrix, which is the weight percent of nickel which forms a solidsolution with tungsten. While liquid solutions of the metals are desiredfor additive purposes, in cathode production certain solutions as thechlorides of the mentioned metals are undesirable.

The above example has been repeated with due changes in the process andmaterials in the prescribed ranges with good results. In addition, thisinvention has been practiced, in accordance with the above example, withsuch additive metals as cobalt, platinum and palladium. For example,about 1% by weight of cobalt was added by means of a known concentratesolution of cobalt nitrate dissolved in methanol. In another exampleabout 1% by weight of platinum was added by means of a known concentratesolution of platinum nitrate in methanol. In a still further example,about 1% by weight palladium was added by means of a solution preparedby dissolving palladium in hydrochloric acid, evaporating the solution,and dissolving the residue in methanol. These latter mentioned metaladditives also provided excellent dispenser cathodes in accordance withthe teachings of this invention, the effects of the additives beingsimilar to those of the preferred nickel additive. In all instances itis preferred to utilize the additives in liquid solution form, and tolimit the additive to about 1 weight percent of the cathode. Wheredesirable, mixtures, including alloys, of the additive metals may alsobe employed.

Various pressing and heating apparatus and cycles may be employed tocarry out this invention. A combined pressing and heating operation in acarbon monoxide or carbon dioxide atmosphere may also be employed, asdescribed in the aforementioned copending application. For example, theoperation may be carried out in the presence of carbon where the carboncan react to produce a protective atmosphere. One apparatus toaccomplish this result is illustrated in FIG. 1. In FIG. 1 apparatus 10includes a carbon crucible 11 defining a cylinder 12. A plunger 13 whichalso may be of carbon is operative in cylinder 11 to compress a cathodemix 14. Simultaneous heating of cathode mix 14 is accomplished byconnecting the apparatus 10 to a suitable source of power (not shown) bymeans of electrical conductors 15 and 16. Material 14 is then heated byelectrical resistance heating through the crucible and material 14 orthrough the crucible alone. Sintering temperature is preferably about1300 C. for best results. Lower temperatures are insuflicient to provideoptimum coherency and strength characteristics. Furthermore, sinteringbeyond about one hour at temperature is unnecessary.

While a particular cathode has been described as an article and by amethod of producing, variation of these will become apparent to thoseskilled in the art without departing from the scope of the disclosedinvention. It is therefore intended by the appended claims to includeall such equivalent variations as come within the true spirit and scopeof the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A method of producing a dispenser cathode which comprises incombination (a) providing a refractory metal in particulate form,

(b) .adding a material to said refractory metal to coat said refractorymetal with an additive metal which is in the range of about 0.3 to 1.0weight percent of said refractory metal, and not substantially more thanwill alloy with said refractory metal,

(0) said additive metal including a metal taken from the classconsisting of those metals of Group VIII of the Periodic Table ofElements,

((1) adding at least about 8.0 weight percent of a barium compound tosaid refractory metal,

(e) adding less than about 1.0 weight percent of CaCO to said refractorymetal,

(f) compressing said mixture at about 10 tons per square inch pressureand (g) firing said mixture in a reducing atmosphere at about 1300 C. toprovide a sintered integral high strength dispenser cathode.

2. The invention as recited in claim 1 wherein said material is a liquidsolution containing a metal of those metals of Group VIII of thePeriodic Table of Elements.

3. The invention as recited in claim 1 wherein said coating material isnickel solution, said barium compound is barium orthosilicate (Ba S' 4)-4. The invention as recited in claim 1 wherein said additive metal iscobalt! 5. The invention as recited in additive metal is platinum.

claim 1 wherein said a 6 6. The invention as recited in claim 1 whereinsaid 2,914,402 11/1959 Becker et a1. 75-202 additive metal is palladium.2,929,133 3/1960 Hughes 75207 7. The invention as recited imclaim 1wherein said FOREIGN PATENTS refractory metal 13 tungsten and sald banumcompound 1s 25,854 Ad). 1908 Great Br1ta1n.

barium orthosilica-te. 5

L. DEWAYNE RUTLEDGE, Primary Examiner.

References Cited by the Examiner REUBEN EPSTEIN, LEON D. ROSDOL,Examiners.

UNITED STATES PATENTS 2,492,142 12/1949 Germashausen 7s 2o7 R. L.GOLDBERG, R. L. GRUDZLEC'KI, 2,700,118 1/1955 Hughes et a1 "75-206Assistant Examiners.

1. A METHOD OF PRODUCING A DISPENSER CATHODE WHICH COMPRISES INCOMBINATION (A) PROVIDING A REFRACTORY METAL IN PARTICULATE FORM, (B)ADDING A MATERIAL TO SAID REFRACTORY METAL TO CAOT SAID REFRACTORY METALWITH AN ADDITIVE METAL WHICH IS IN THE RANGE OF ABOUT 0.3 TO 1.0 WEIGHTPERCENT OF SAID REFRACTORY METAL, AND NOT SUBSTANTIALLY MORE THAN WILLALLOY WITH SAID REFRACTORY METAL, (C) SAID ADDITIVE METAL INCLUDING AMETAL TAKEN FROM THE CLASS CONSISTING OF THOSE METALS OF GROUP VIII OFTHE PERIODIC TABLE OF ELEMENTS, (D) ADDING AT LEAST ABOUT 8.0 WEIGHTPERCENT OF A BARIUM COMPOUND TO SAID REFRACTORY METAL, (E) ADDING LESSTHAN ABOUT 1.0 WEIGHT PERCENT OF CACO3 TO SAID REFRACTORY METAL, (F)COMPRESSING SAID MIXTURE AT ABOUT 10 TONS PER SQUARE INCH PRESSURE AND(G) FIRING SAID MIXTURE IN A REDUCING ATMOSPHERE AT ABOUT 1300*C. TOPROVIDE A SINTERED INTEGRAL HIGH STRENGTH DISPENSER CATHODE.